1. Field of the Invention
This invention relates to a wind turbine and more particularly to a wind turbine employing articulating blades and overspeed control. Wind turbines of the type to which this invention relates are most usually operated with a vertical axis, although a horizontal axis form is possible. The blades of the wind turbine are automatically adjustable with respect to the relative wind while the blades rotate about the central axis.
Throughout the description of this invention, there will be frequent reference to wind turbines, it should be understood however that the turbine contemplated responds to any moving fluid stream and the invention is to be interpreted accordingly.
2. Prior Art
Vertical axis wind turbines are potentially simpler, lighter, and less expensive than other wind energy conversion systems. The ideal vertical axis wind turbine would achieve minimum weight and minimum complexity combined with maximum efficiency, so as to convert wind energy at the lowest cost. The prior art has been only partially successful, due to many interactive problems. The dominant problem is that of the very high centrifugal bending stresses which act on the vertical blades. Attempts to resolve this problem have engendered numerous secondary problems.
There are three general approaches to the problem of high centrifugal force. These approaches include (1) the use of curved blades so that blade stresses are primarily tensile; (2) the rolling, pitching, yawing, etc., of non-articulating, straight blades so as to limit centrifugal stresses; and (3) the use of blade articulation systems in order to operate efficiently at lower blade speed ratios, since centrifugal force is proportional to the square of the blade speed.
The Darrieus rotor (U.S. Pat. No. 1,835,018) exemplifies the first approach. The non-articulating, curved blades operate at a high speed ratio (6) in order to keep the angle of attack of the blades within reasonable limits. The secondary problems created by this approach include a lack of starting torque, and poor torque at low and moderate blade speed ratios. Additional components, such as a starting motor, an external power supply, a clutch, a brake, wind speed sensors, and control circuitry are required. The curved blades must be held to close tolerances to avoid excessive aerodynamic drag, and blade production therefore requires capital intensive production techniques. The curved blade configuration also limits the options for tower designs. Since the efficiency of the Darrieus rotor is only moderate, the added costs created by these secondary problems have prevented the Darrieus rotor from achieving a lower cost for wind energy conversion.
While one embodiment of the present invention includes the use of curved blades, that configuration would be limited to use under extreme weather conditions. Since that embodiment of the present invention operates at a moderate blade speed ratio (3), its cutout wind speed would be higher than that of the Darrieus rotor. Therefore, its annual energy output would be greater and, in turn, the cost of wind energy conversion would be less. The present invention does not require the use of additional components such as a starter motor, clutch, etc., since the present invention achieves high starting torque. Over-speed control is also inherent in the present system.
A second approach to the problem of centrifugal force is to use non-articulating, straight blades, and to permit the blades to deviate from their normal operating positions in order to limit the centrifugal force acting upon them. There are many obvious ways in which the non-articulating blade may deviate from their normal positions, such as rolling, pitching, yawing, distorting, extending, and variations or combinations of these. The Musgrove rotor (U.S. Pat. No. 4,087,202) employs blade rolling; the Bolie rotor (U.S. Pat. No. 4,204,805) also employs blade rolling, and blade yawing; and the Loth rotor (U.S. Pat. No. 4,105,363) employs blade pitching.
Blade rolling (Musgrove and Bolie) reduces blade bending stresses without necessarily reducing rotor rpm, whereas blade pitching (Loth) and blade yawing (Bolie) limit bending stresses by limiting rpm. Rolling, pitching and yawing all reduce the efficiency of non-articulating blades. Non-articulating, straight blades, which must operate at a high blade speed ratio, cannot be operated at full efficiency during high energy winds (over 15 mph) because the centrifugal bending stresses would exceed the strength of the blades. Consequently, these machines are forced to "spill" energy. The loss of energy can be large because wind turbines typically convert about 75% of their annual total energy during high energy winds. (Power is proportional to the cube of the wind speed.) These machines have little or no starting torque, and they require additional components for starting, etc. While it would be possible to add sufficient external bracing to enable the blades to withstand bending stresses, the aerodynamic drag induced by such bracing would be excessive. These secondary problems have prevented such machines from lowering the cost of wind energy conversion.
The turbine of the present invention is able to operate efficiently at a moderate blade speed ratio (3), and blade stresses are therefore considerably reduced. Consequently, the present turbine is able to operate at full efficiency throughout its normal operating range of wind speeds. For the present turbine, rolling, pitching, etc., of the blades need not commence until the turbine has reached its full rated power.
When rolling, pitching, etc., do commence, the blades of the present turbine simultaneously continue to articulate. The blades actually undergo a complex motion which is neither rolling, etc., nor articulating, but which may be described, for convenience, as a combination of such motions. The turbines of Musgrove, Bolie, and Loth would cease to function altogether if their blades were permitted the freedom of movement permitted the blades of the present turbine. In addition to the numerous structural differences between the present invention and the prior art of Musgrove, Bolie, and Loth, it is important to note that the underlying principles of over-speed control are fundamentally different as well. For instance, blade rolling as used in the Musgrove rotor is not the same as blade rolling as used in the present turbine. The blade motions are fundamentally different for the two turbines, even though the static representations of blade rolling tend to obscure the dynamic differences.
A third approach to the problem of centrifugal force is to articulate the blades, thereby permitting a turbine to operate at low or moderate blade speed ratios without sacrificing efficiency. Blade articulation also permits self-starting of the turbine. Blade articulation may be accomplished using either a fixed rock schedule or a variable rock schedule. A fixed rock schedule consists of a predetermined set of cyclic blade angles, which can be optimized only for a single blade speed ratio. A variable rock schedule consists of cyclic blade angles which vary with the blade speed ratio. Quite different rock schedules are required for low versus high blade speed ratios. Small deviations from an optimum rock schedule cause relatively large reductions in rotor efficiency. Since wind velocities fluctuate rapidly and continuously, wind turbines experience rapidly and continuously fluctuating blade speed ratios. Therefore, variable rock schedules are potentially more efficient than fixed rock schedules.
Darrieus (U.S. Pat. No. 1,835,018) shows a blade articulation system for lift-type vertical axis turbines. His turbine employs an eccentric with pull rods to achieve a fixed rock schedule. The articulating Darrieus rotor is relatively inefficient, however, since the resulting rock schedule is sinusoidal and therefore inappropriate.
The Dress rotor (U.S. Pat. No. 4,180,367) employs a carefully designed cam, spring-loaded pull rods between the cam and the blades, and a tail vane to orient the cam to the wind, to achieve a moderately efficient fixed rock schedule. However, since the turbine must overcome its own inertia in order to operate at a constant blade speed ratio, rotor efficiency drops off considerably in turbulent winds. This handicap, combined with the complexity and cost of the articulation system, has prevented the Drees rotor from lowering the cost of wind energy conversion.
The Giromill (no patent), developed by the McDonnell Aircraft Co., employs a variable rock schedule for blade articulation. The basic elements of the articulation system include wind sensors, feedback circuits, a computer, and electromechanical blade actuators. The turbine is quite efficient, but the high efficiency is offset by the considerable complexity and high cost of the articulation system.
The Sicard rotor (U.S. Pat. No. 4,048,947) achieves a variable rock schedule by making use of the equilibrium between opposing aerodynamic and centrifugal forces, as does the turbine of the present invention. However, the Sicard rotor uses flyweights attached to the blades in order to create the centrifugal restoring force for the blades. The use of blade flyweights increases the inertia of the counterweighted blades, and thereby reduces their responsiveness to the aerodynamic articulating forces. Although the flyweights may, alternatively, be combined with the blade counterweights if the counterweight arms are angled outward, this combination is arbitrary and does not serve to decrease the excessive blade inertia; the total weight and weight distribution of the blades remain unchanged. The Sicard rotor is handicapped by excessive blade inertia, since rotor efficiency is dependent upon the ability of the blades to respond instantly to continuous changes in both the direction and the velocity of the relative wind. Small deviations from an optimum rock schedule cause large reductions in rotor efficiency. The Sicard rotor also has poor starting torque, which is the result of allowing the blades freedom to rotate a full 360 degrees around their articulation hinges.
The turbine of the present invention does not employ the use of blade flyweights, or their equivalent, for blade articulation. Instead, the blade assemblies create their own centrifugal restoring force as a result of mounting the blades and their counterweights at the outward end of their respective rocking arms. This configuration is simpler, lighter, and more efficient than the Sicard system. The present turbine also achieves high starting torque due to limiting maximum rock angles to approximately 45 degrees. The blade assemblies of the present invention require no additional mechanisms in order to achieve a variable rock schedule, and the present turbine is able to operate at full efficiency in high energy winds.
The novel solution embodied in the present invention is the manner in which centrifugal force, itself, is used to circumvent the problems that would otherwise be imposed, directly or indirectly, by centrifugal force. None of the prior art mechanisms, of which I am aware, has suggested the lightweight, self-articulating blade assembly with inherent over-speed control and multiple speed control options as herein described, in order to simultaneously achieve simplicity, efficiency, versatility, and low cost.